It is estimated that over 90% of chemical processes use a catalyst, with 80% being a heterogeneous catalyst, with a global demand of $15 to $20 billion per year. Microporous materials (pores less than 2 nm) are an important type of heterogeneous catalyst as they offer shape and size selective environments for catalysis to occur. Additionally, they often exhibit robust hydrothermal stability which allows them to be used under demanding process conditions, such as fluid catalytic cracking. Synthetic aluminosilicate zeolites are produced on a scale 1.7-2 million metric tons per year, and their use as catalysts comprises 27% of the world market for zeolites. As the cost of the catalyst is estimated to be only 0.1% of the cost of the final product, the demand to innovate in this area remains high. There currently exist over 200 known microporous material frameworks, but of these less than 20 have been commercialized and the market is dominated by only five major frameworks. In many applications, there is only a single structure and composition to achieve optimal performance, motivating much of the research directed at creating new materials.
In recent years there has been considerable interest in 8-MR systems for catalysis and separations. Some of the most promising catalytic applications are the methanol to olefins (MTO) conversion and deNOx. Other 8-MR materials of interest are LEV, CHA and AFX. It has been found that the cage size and connectivity are critical in determining the product distribution for these reactions in 8-MR systems. As RTH possesses a unique connectivity as well as cage size it exhibits unique catalytic performance, which has been shown for MTO using the aluminosilicate material. The RTH topology has 8-MR openings and a 2-dimensional (“2-D”) channel system with pore sizes of 4.1×3.8 Å and 5.6×2.5 Å, leading to larger cages. See also C. Baerlocher, L. B. Mccusker, Database of Zeolite Structures, approved by the Structure Commission of the International Zeolite Association, and available at <http://www.izastructure.org/databases/>, (2014).
The molecular sieve (zeolite) with the framework topology of RTH was first described in 1995. It was produced as a borosilicate (RUB-13) using the relatively simple organic structure directing agent (“OSDA”) 1,2,2,6,6-pentamethylpiperidinium. Although the borosilicate requires a less exotic OSDA, the small pores of the material prevent the replacement of boron with aluminum. However, in order to produce an aluminosilicate material with the necessary acid strength for the MTO reaction, a more complicated OSDA was required. The next RTH microporous material was produced as an aluminosilicate (SSZ-50) using 2-ethyl-2,5,7,7-tetramethyl-2-azabicyclo[4.1.1]octan-2-ium as the structure directing agent.
But this OSDA requires an elaborate multi-step synthesis adding to the cost and complexity of the preparation, so as to be unlikely this material would be used in commercial production. Some progress has been made to synthesize a OSDA-free version of RTH using seed crystals, but materials so-produced have a very limited compositional range. Additionally, the high Si/Al ratios (Si/Al=41 and 108) may be less than optimal for a catalyst. For all of these reasons an alternative SDA to produce RTH is desired so that it can be tested for applications.
Microporous solids having 2-dimensional channels and 8-MR and 8-/10-MR frameworks are finding increasing utility as catalysts, ion exchange, and adsorption systems. The heulandite (HEU) framework is another topology having a 2-dimensional channel system, and relatively large pore sizes. In the [001] directing there are 10-membered rings (MRs) as well as 8-MRs. Additionally, there is another set of 8-MRs along with [100] direction.
Heulandite materials are divided into two distinct classes based on Si/Al ratio. Those with Si/Al of less than 4 are known as heulandite and those with Si/Al greater than 4 are known as clinoptilolite, or silica-rich heulandite. The key difference in these materials is that those with Si/Al of less than 4 are not thermally stable to calcination above 350° C. A method to produce a high-silica heulandite was first reported by removing aluminum from natural clinoptilolite to make a material with Si/Al=5.5, known as LZ-219. Later, methods were developed to produce synthetic heulandite across a range of Si/Al=2.5-6 using various sources of silica and alumina and a wide range of inorganic cations (Li, Na, K, Rb, Ca) under hydrothermal synthesis (without the use of OSDAs) or acid leaching conditions. These materials have been considered for applications such as gas cleaning and separations, ion exchange, isomerization of 1-butene and xylene, methanol dehydration and acetylene hydration. In any of these applications, the ability to tailor the framework composition is important for material properties such as exchange capacity, hydrothermal stability and pore accessibility.
Crystalline microporous solids of the IWV framework were first prepared as the ITQ-27 framework using the phosphorus-containing structure-directing agent, dimethyldiphenylphosphonium. Its structure comprises seven unique T-sites forming a framework with straight 12-MR channels that are connected by 14-MR openings between them. Since access from one 14-MR opening to the next is through the 12-MR channel, the structure is best described as a two-dimensional, 12-MR framework. Other ITQ structures are known to have larger (e.g., 12-/14-MR) and smaller ring openings, making these materials useful for hydrocarbon processing.
Much of the discovery of new microporous material frameworks and compositions in recent years has resulted from the use of organic structure directing agents (OSDAs), which are normally alkylammonium cations. While OSDAs have led to many new materials, their cost contributes a significant portion of the material cost, which often cannot be recovered as they are normally removed from the material using combustion. In some systems it is possible to partially replace high cost OSDAs with cheaper organics, such as with SSZ-13 where it has been shown that over 80% of the expensive trimethyl-N-1-adamantammonium hydroxide OSDA can be replaced with the much cheaper benzyl trimethyl ammonium hydroxide. Another way is to find methods to synthesize the materials in the absence of OSDAs, but this can often lead to limited product compositions and still does not eliminate processing steps such as ion exchange and calcination. Therefore, an attractive route to lower OSDA costs is to find new, simpler OSDAs to synthesize desired materials.
For at least these reasons, interest is currently high in developing new molecular sieves having 2-dimensional channel systems with 8-MR and 8-/10-MR openings and 12-MR openings.